The present invention relates to a braking force modulator for insertion into hydraulic brake fluid conduits in vehicles equipped with hydraulic brakes to form, together with suitable devices for sensing the rotational state of the vehicle wheels, a complete system to prevent wheel-locking during application of braking forces which exceed the limit imposed by the momentary state of friction between a vehicle wheel and the road surface.
Basically, complete so-called "anti-lock" or "anti-skid" systems of this type operate in such a way that when the device (the wheel sensor) monitoring the state of rotation of a vehicle wheel during braking detects a tendency of the wheel to be retarded beyond a certain established limit, the modulator immediately blocks additional accumulation of hydraulic pressure in the wheel brake cylinder in question. The modulator also in some way evacuates hydraulic oil or fluid from the wheel brake cylinder so that the braking moment is reduced. The retardation of the vehicle wheel is thus reduced and and the wheel begins to accelerate. When the vehicle wheel has achieved a speed approaching the speed which a non-braked wheel would have at the actual vehicle speed, the wheel sensor may change the signal given to one which may be an order to the modulator to increase the hydraulic oil pressure or to maintain it on a constant level.
As the present invention is only concerned with a braking pressure modulator there is no reason to deal in greater detail with wheel sensors which are available in many known constructions, both mechanical and electronic, representing different control philosophies. Some of these sensors emit only two signals: either (1) to stop additional pressure accumulation and to lower the braking force, or (2) to increase the braking force. As indicated before, there are also wheel sensors which in addition to the two signals mentioned above may also signal that braking force should be maintained at a constant level.
As the present specification is concerned with a modulator for hydraulic braking systems the expressions "braking force" and "braking moment" used above are substantially synonymous with hydraulic oil or brake fluid pressure in a wheel brake cylinder, because the braking moment in a hydraulic brake essentially is directly proportional to the hydraulic pressure applied. In this specification the expression "brake fluid" ordinarily used by the experts on the field is used irrespective of the kind of hydraulic fluid actually used.
It will appear from the above considerations that a universally useful modulator, i.e. a modulator which can be used in combination with any wheel sensor known so far, must fulfill with a requirement to be able to receive and carry out four orders, namely:
1. Bar pressure accumulation in the wheel brake cylinder.
2. Reduce the pressure in the wheel brake cylinder.
3. Increase the pressure in the wheel brake cylinder.
4. Maintain the pressure in the wheel brake cylinder at a constant level.
The patent literature contains a great number of descriptions of braking modulators, some of which can only receive and respond to the first three of the above indicated orders. In addition to being complicated, sensitive to disturbances and expensive they have a common deficiency in that they are difficult to bleed. They contain solenoid valves and passageways in which air can be trapped so that it cannot be removed by the ventilating principle well-known to the experts and practised for many decades, namely simply to open a ventilating nipple on the wheel brake cylinders. Ventilating nipples must also be provided on the modulator. In certain cases an exact procedure must be followed which often requires actuation of solenoid valves during certain phases of the procedure. With bleeding procedures being extremely difficult to such systems, deficient ventilation in certain of the previously known modulators involves a direct danger due to the fact that, if the prescribed ventilating procedure is not minutely followed, air may be trapped within the modulator which does not interfere with the normal function of the brake. The mechanic or service man may have bled the brake system until the "pedal sponginess" characteristic of a brake system containing a compressible gas has been eliminated. This may mean that, when the vehicle is involved in a situation requiring the modulator to come into action, the modulator will be inoperative due to the trapped air. There are even examples of modulators which, when actuated while air is trapped in the modulator, release air into the normal braking system causing even that system to become inoperative.
Thus it is a reasonable requirement of a modulator that a system including the modulator may be bled or ventilated without requiring additional measures beyond the established procedure which simply involves an opening of the "ordinary" ventilating nipple in the wheel brake cylinder. Only two modulators are described in the patent literature which at least to some extent fulfill this requirement, namely a modulator according to Swedish Pat. No. 75 01883-8 and a modulator according to U.S. Pat. No. 4,138,165.
Briefly, these modulators may be described as a conventional piston pump, the piston of which is oscillated by means of an eccentric driven by an electric motor. The pump is provided with two spring-biased check valves, one of which serves as an inlet valve and the other as an outlet valve. Normally the check valves are held open by a spring, permitting brake fluid freely to pass through the modulator in both directions during normal braking. Disposed between the two check valves is a solenoid which, upon energization, is able to overcome the force of the spring which normally holds the two check valves open and thereby bar further increase in brake fluid pressure. When the piston-moving mechanism starts, the pump starts pumping brake fluid from the wheel cylinder back to the braking fluid pressure generator, ordinarily a foot-operated master cylinder with a piston which generally is provided with a so-called vacuum servo mechanism reinforcing the power exerted by the foot of the operator. This happens when the wheel sensor signals that brake fluid pressure in the wheel brake cylinder should be reduced. When the current supply to the solenoid magnet is interrupted, the two check valves are opened again, permitting brake fluid to flow from the master cylinder through the modulator and to the wheel brake cylinder, thus causing the pressure therein again to be increased. This happens when the wheel sensor signals that the pressure in the wheel brake cylinder should again increase. The modulator according to Swedish Pat. No. 75 01883-8 is a fundamentally simple device which obviously may be manufactured at a reasonable cost and which at first sight appears to satisfy all requirements to be met by a braking modulator for hydraulic vehicle brakes, such as the requirements regarding simple ventilation as discussed above.
However, upon closer analysis, the modualator may be found to exhibit a number of deficiencies such that it cannot satisfy the requirements which a braking modulator must meet in order to be acceptable at this time. These include:
(1) System bleeding or ventilation is not entirely free of problems as previously indicated by the choice of the words "at least to some extent fulfill this requirement". Due to the fact that the two check valves are opened and closed by one solenoid device, a valve chamber must be provided which is connected to the space above the pump piston by means of a duct. When bleeding is performed, brake fluid mixed with air bubbles flows through the valve chamber but not through the space above the piston, which thus will have to be ventilated by means of brake fluid flowing under the action of gravity downwardly through the duct connecting the two spaces. The duct must be rather wide if the air in the space above the piston is to pass upwardly through the duct to the valve chamber to be discharged therefrom during bleeding and leave the modulator. If the duct is too narrow, air bubbles may, due to the high surface tension of the brake fluid, prevent the air from leaving the space above the piston. Obviously, bleeding a system using the modulator of Swedish Pat. No. 75 01883- 8 may be problematical.
(2) From Section 1 above it appears that in order to enable the modulator to be ventilated at all it must be oriented in a certain way, namely with the valve chamber disposed substantially directly above the space overlying the piston. This condition is difficult to satisfy in the narrow spaces available for mounting a modulator in modern vehicles.
(3) The duct mentioned in Section 1 and connecting the valve chamber and the space above the piston is an inconvenient dead volume which may endanger the function of the pump. Brake fluid has a considerable elasticity which, in pumps having small stroke volumes in relation to unavoidable dead volumes as is the present case, has the effect of preventing the pump from producing large pressure differentials.
(4) A single solenoid operating two check valves must be disposed between them which means that the valve forming the outlet valve of the pump and nearest the master cylinder of the braking system must be operated by the solenoid by means of a pin passing through the valve seat. The effective flow area of the valve then amounts to the difference between the cross-sectional areas of the valve seat and the operating pin. The brake fluid pressure differential operates against the cross-sectional area of this pin, but the area does not contribute to the flow area of the valve. Even if a pin is used having a diameter as small as 1.5 mm, this means, at a pressure differrential of 150 bar, a force of 27 Newton. When the valve is opened this force is a net loss and constitutes an additional requirement on the solenoid which therefore will be large and accordingly slow.
(5) As the two check valves are operated by a single solenoid, the modulator cannot receive and execute the last one of the four types of orders mentioned previously. To do so requires that the two check valves be operated independently of each other. Theoretically this order "hold the pressure in the wheel braking cylinder at a constant level" might be carried into effect by holding the solenoid energized (and thereby the check valves closed) and stopping the motor. In reality, however, this is not an operable solution because the inertia of the motor will cause the motor to continue moving for some time in spite of the fact that the current supply has been interrupted. There are established methods to shorten the deceleration time of motors subsequent to current interruption, e.g. by short-circuiting the motor causing it to switch-over to generator-action producing a braking moment. It is also possible, after having interrupted the current, to transmit a short current pulse backward through the motor in order to bring it to a standstill. The motor may also be braked by a solenoid operated spring-biased brake which is held released by the same current which is fed into the motor. This last-mentioned measure may be combined with those mentioned previously. However, these measures are not sufficient in order to produce the required short reaction times.
To bring about a quick response to wheel sensor signals, the motor preferably should be in action during the complete period of time in which the system comprising the wheel sensor and the modulator is controlling a braking sequence. The reason therefore will appear from the description of the present invention given hereinafter.
(6) The braking modulator according to Swedish Pat. No. 75 01883-8 lacks any mechanism enabling the sensor order "increase the pressure in the wheel brake cylinder" to be executed in a controlled way. When the solenoid opens the two check valves, the brake fluid pressure will be immediately increased. This produces considerable problems as far as control technique is concerned, and the system would act jerkily while the heavy applications of brake fluid pressure would expose the suspension of the vehicle to violent loads. To operate in a satisfactory way, the modulator must be complemented by means enabling the pressure increase derivate to be controlled. A number of such devices are disclosed in Swedish Pat. No. 76 05863-5.
(7) Practical tests have shown that, without interference into the normal function of the braking system, restrictions having very small open areas may be inserted into brake fluid conduits. Thus, it would be possible to give the two check valves a very small cross-sectional area which would mean low operating power requirements for the solenoid which thus could become small, economic in current consumption and quick.
This is opposed by a requirement of a considerably larger cross-sectional area for enabling the check valves to operate as efficient inlet and outlet valves during pumping.
It has been found that braking systems even for heavy passenger cars permit restrictions in the form of openings having a diameter of less than 1 mm, with 0.7 mm being common and a still lower value possible for smaller cars. A hole having a diameter of 0.7 mm has an area of 0.38 mm2.
The maximum brake fluid pressures in modern braking systems are never lower than 120 bar, 150 bar being an ordinary value and the maximum pressure in extreme cases amounting to 200 bar and more. On an area of 0.38 mm2, a pressure of 150 bar yields a force of 5.7 Newton which can be rather easily opposed by a rather small solenoid.
Opposed to this is the fact, established by practical tests, that in order for the pump to reach a good pumping effect and a reasonable efficiency, much larger valves are required. For normal passenger cars the requirement is a diameter of at least 3 mm and preferably 4 to 5 mm. A brake fluid pressure differential of 150 bar yields in valves of 3 mm diameter a force exceeding 100 Newton. It is not possible to produce this force even with reasonably large solenoids, and the problem is made more difficult by the requirement of extremely short reaction times.
From the above explanation it will appear that a person trying to construct a serviceable modulator according to Swedish Pat. No. 75 01883-8 will be forced to find a compromise between the two quite incompatible requirements explained above regarding the diameters of the check valves, which will be a difficult or almost impossible task.
The modulator according to U.S. Pat. No. 4,138,165 has the same deficiencies as those explained in Sections 1-5 in connection with the modulator according to Swedish Pat. No. 75 01883-3. Those described in Sections 6 and 7 have been eliminated in the modulator according to the United States Patent by constructing the two check valves as double valves. Such a valve comprises a relatively large valve disk in which a smaller valve is provided. When the modulator is not in action, both the large and small valves are held open. When the modulator is conditioned to obstruct continued passage of brake fluid, both the large and smaller valves are closed, and during pumping the large valves act as inlet and outlet valves.
When the brake fluid pressure is to be restored, the solenoid return spring is not able to open the two large valves due to the pressure differential across them. In contrast, the two small valves are opened and the area difference between a central hole in the large valves and an actuating pin for the small valves extending tending therethrough constitutes the restriction required for a soft, controlled re-application of the brake. When the brake fluid pressure across the modulator has been substantially completely equalized, the large valves open. It will be seen that the two deficiencies described in Sections 6 and 7 above have been at least theoretically eliminated. Sufficiently large pumping valves have been obtained as well as a controlled restriction of the brake fluid flow upon renewed application of the brake.
Attempts to develop mass produced modulators according to this United States Patent have shown that the construction comprises a large number of small, very intricate detail parts which must be manufactured at prohibitively small tolerances. In addition, the fact that a single solenoid operates both the inlet and outlet valves requires that the range of manufacturing tolerances be very carefully monitored, which in turn requires that parts made to conventional tolerances must be separated by measurement into classes for selective assembly. In assembly, it has been shown in practice that in spite of very exact manufacture of the detail parts and thorough selection-mounting the device had to be dismantled and parts exchanged several times in order to achieve proper function.
Practical tests designed to find a suitable restriction area for the brake fluid upon renewed application of the brake during a controlled braking process have shown that the restriction area should be very small. This means that the gap between the operating pin for the smal valve and the hole in the large valve will be so small that there is an obvious risk that the gap will be reduced after some time due to the collection of oxides, dirt and other deposits, causing the restriction area to be diminished and the function of the modulator to be endangered. If the restriction area becomes excessively small, the brake is re-applied so slowly that the braking distances are extended during a controlled braking process. The consequences would be completely disastrous if the gap were entirely clogged.
On the basis of the above remarks and after having studied the drawings of U.S. Pat. No. 4,138,165, the expert will realize that this patent does not form an acceptable starting point for developing a reliable and sufficiently cheap product suitable for mass production.